The following discussion of the background to the invention is intended to facilitate an understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge as at the priority date of the application.
Photovoltaic (PV) cells that convert sunlight directly into electricity are becoming increasingly important in the world's renewable energy mix. Currently, around 85% of PV cells have a photoactive element based on crystalline Si, with the rest being polycrystalline thin film PV cells, mostly cadmium telluride/cadmium sulfide ones. Thin-film cells tend to be cheaper to make with a shorter energy payback time. A rapidly developing newcomer to the thin film PV field is based on organic-inorganic perovskite-structured semiconductors, the most common of which is the triiodide (CH3NH3PbI3). Such Perovskites tend to have high charge-carrier mobilities and therefore make ideal photoactive components.
Large scale production of these types of PV cells is difficult because the process of applying the photoactive layer has been found to be difficult to scale. Currently, spin coating has been used at the lab scale to coat a perovskite precursor solution onto a substrate. Spin coating allows for the formation of very homogenous films over an area up to 300 mm in diameter. In a spin coating process, a liquid is applied to a substrate prior to or after acceleration of the substrate to a chosen speed. The substrate spins rapidly and solution on the substrate spreads and dries. Therefore, formation of large size crystals can be minimized or controlled. However, it is not possible to use spin coating on a larger scale. In a scalable coating/printing process, for example dipping or wet coating, a wet film of coating material is formed first and then dried. This has a significantly different solution dynamic and drying time compared to spin coating. In wet coating, the wet film is susceptible to dewetting, non-homogenous crystal formation and/or the formation of pinholes, all of which have undesirable effects on the function of the photoactive layer. These problems become increasingly more serious the greater the amount and area to which the crystalline material is applied.
It is therefore desirable to provide a new and/or improved process or method of forming a photoactive layer of a perovskite photoactive device such as a solar cell.